The present invention relates to fluid separation devices for use in vent volumes within a nuclear fuel bundle and particularly to devices for flowing liquid from a path through the vent volume laterally outwardly into the interstices between and onto adjacent surrounding fuel rods with minimum pressure drop.
A typical boiling water nuclear reactor has a reactor core comprised of a plurality of fuel bundles in side-by-side relation to one another. Coolant/moderator flows upwardly within the fuel bundles and about the fuel rods within the fuel bundles where the liquid is converted to steam to produce power.
In U.S. Pat. No. 5,112,570 there is illustrated a fuel bundle having a plurality of part-length fuel rods (PLR). These PLR""s are supported on the lower tie plate of each bundle and extend upwardly toward the upper tie plate. The rods, however, terminate short of the upper tie plate and typically between a pair of spacers along the fuel bundle. Between the upper end of each PLR and the upper tie plate, there is defined in the upper two-phase region of the fuel bundle a vent volume. This vent volume preferentially receives vapor from the liquid vapor two phase mixture in the upper region of the fuel bundle during power producing operations. There are many advantages associated with the use of PLR""s including the increased vapor fraction within the vent volume and the pressure drop reduction in the upper two phase region of the bundle. This results in increased stability from thermal hydraulic and nuclear instabilities.
It will be appreciated that the mechanical hardware associated with fuel rod spacers causes local reduction in the flow area available for the vapor and liquid flowing through the fuel bundle. This causes significant pressure drops to occur as the flow passes each spacer. By using PLR""s, the associated flow blockage effects of one or more of the full-length fuel rods extending through these spacers above the PLR is substantially eliminated. That is, because of the absence of a fuel rod at a lattice location above one or more PLR""s, more flow area through the spacer is obtained with consequent reduction in pressure drop across such spacer. Further, significant flow diversion occurs into the lower pressure drop paths or vent volumes above the upper ends of the PLR""s. Thus, increased vapor and liquid are pumped from surrounding flow passages, i.e., the interstitial regions around the adjacent fuel rods, into these vent volumes.
The creation of vent volumes, e.g., above PLR""s, and flow diversions resulting therefrom, however, can cause some reduction in critical power performance in the fuel bundle. Additional water may accumulate in the vent volume region above the PLR and thus be shunted out of the vent volume without heat generating contact with the remainder of the full-length fuel rods. Separation devices have been utilized to drive the dense liquid or water out of the vent volumes in a generally lateral direction onto the surfaces and into the interstitial regions between the full-length fuel rods to improve heat transfer performance. Such separation devices have generally taken the form of swirlers disposed in the vent volume. These swirlers create a helical flow pattern causing the dense liquid to be driven laterally outwardly of the vent volume by centrifugal force. Such separation devices have been located within the spacers and have extended therefrom above or below the spacers or to both sides of the spacers. By locating the separation devices within the spacers, however, the devices increase the pressure drop across the spacers substantially to the same extent as if full-length fuel rods occupied those lattice positions in the vent volumes. Thus, the value of introducing PLR""s in reducing the pressure drop along the length of the fuel bundle is minimized or eliminated by using conventional separation devices in the spacers above the upper ends of the PLR""s which restore in part or in whole the pressure drop achieved by the use of PLR""s.
According to the present invention, there is provided a fuel bundle for a nuclear reactor having a vent volume wherein one or more separation devices are used to direct liquid laterally onto the surfaces and into the interstices of the full-length fuel rods but without a substantial increase in the pressure drop across the spacers. By placing the separation device(s) above a spacer and leaving the opening(s) through the spacer at that lattice position(s) of the vent volume void of fuel rods, e.g., substantially unobstructed, maximization of the flow through the spacer at a minimum pressure loss while simultaneously directing additional liquid for deposition onto the surfaces of the adjacent full-length fuel rods can be achieved. The location of each separation device above a spacer thus maximizes flow diversion without substantial increase in pressure drop. That is, maximum benefits from depositing the liquid onto adjacent full-length fuel rods with minimum pressure drop are achieved by locating the separation devices just above the spacers. In addition, by streamlining the spacers and separation devices, the pressure drop can be minimized along the length of the vent volume.
In a first preferred form of the present invention, the separation device is located on top of a spacer within a vent volume and may comprise a swirler. It will be appreciated that the separation device in a broad sense need only deflect or divert the liquid flowing upwardly in the vent volume laterally outwardly onto the surfaces and into the interstices of laterally adjacent fuel rods. For that purpose, flow directing devices such as tabs, vanes and the like may be used. Thus, the separation device may have a lower end just above an opening through the spacer (the opening being in a lattice position which would otherwise have supported a fuel or moderator rod) and extend a short distance or an extended distance toward the next adjacent upper spacer.
Where the separation devices comprise the preferred swirlers, each swirler may consist of a single strip of material twisted to form a helical flow path in the vent volume sufficient to direct the heavier liquid laterally outwardly by centrifugal force onto the surfaces and into the interstices of laterally adjacent fuel rods. A more complex configuration may be provided with two or more twisted strips joined along their axes. For example, two flat strips may be slotted at opposite ends, joined along their axes and twisted with the strips maintained perpendicular along their length establishing a cruciform cross-section at any axial location. This can be characterized as a four-blade swirl device. If three strips are joined and twisted with 60xc2x0 angles maintained along their length, a six-blade swirl device is established. It will be appreciated that the minimum length requirement for a swirler decreases as the number of blades are added to the swirl device. However, the surface area for friction increases with the number of blades and hence a swirl device with minimum length to minimize pressure drop is desirable. Hence, the minimum length for effective separation is that which results in a projected area covering a full 360xc2x0 which in turn is a function of the number of strips and the angle through which the strips are twisted. For a single twisted strip, this requires a length equal to 180xc2x0 of rotation. With a double strip configuration of a four-blade swirl device, the minimum length required is equal to 90xc2x0 rotation, while a six-blade swirl device requires a length equal to 60xc2x0 of rotation. In general, the minimum length required for any multi-blade swirl device is that which produces blade rotation equal to 360xc2x0 divided by the number of blades thereby providing a swirl device length which minimizes pressure drop.
Additionally, the separation device may comprise an auger configuration to cause helical flow in the vent volume. Thus, one or more strips of material may be wound on its edge around a central shaft. This type of separation device may extend through one or more spacers with the auger blading extending full length on the shaft or at intervals along the shaft to locate the helical flow pattern producing part of the device just above each spacer. It may also be removable through the upper tie plate.
In another form of the present invention, it will be appreciated that twisting or winding of strips typically results in configurations with horizontal projected areas that are circular. When the separation devices are used in vent volumes, the square pitch of the fuel bundles caused by the removal of one or more fuel rods from one or more fuel bundle lattice position(s) creates a flow passage area which is rectilinear, i.e., more square than circular. Using helical flow devices with circular projected areas in each vent volume produces inefficient swirl flow patterns because of the flow which bypasses the circular cross-section. In accordance with the present invention, significant performance improvement is obtained by shaping the perimeter of the separation device such that the resultant projected area is in general conformance with the shape of the vent volume. For example, by forming the separation devices, e.g., a swirler, with outer dimensions initially larger than the flow passages and then machining it to final shape to match the cross-sectional area of the vent volume, the bypass passages may be eliminated.
It is also advantageous for a helical flow pattern to persist as far as possible toward the next spacer in order that liquid may continue to be fed onto the adjacent fuel rod surfaces. This can be assured by extending the swirl device further toward the next spacer. This, however, involves trade offs because of the adverse pressure drop created by the extended swirl device. The adverse pressure drop can be mitigated by use of a non-uniform swirl device. For example, the aggressiveness of the swirl device can be reduced by changing its pitch or diameter, i.e., reducing its diameter or increasing pitch in a vertical upward direction.
The separation devices can be permanently attached to the fuel rod spacers. This provides high performance and reliability. However, such permanent attachment precludes easy removal of underlying PLR""s. Thus, the separation devices may be individually attached to a spacer as removable devices or may be attached in groups-to a removable central shaft or other structural support. The structural support may carry multiple-blades of twisted strip devices as well as auger devices.
As stated previously, separation devices which create helical swirl flow patterns can be placed above one or more PLR""s in a fuel bundle lattice. When more than one such swirl flow device is utilized, the swirl devices may effect flow rotation in the same direction or different directions of flow rotation. Additionally, the separation devices are preferably placed above all fuel rod spacers in vent volumes, e.g., those created by overlying one or more PLR""s. However, in many designs the local power sufficiently reduces at the top of the fuel bundle that separation devices are not required above the uppermost fuel rod spacer.
In a preferred embodiment according to the present invention, there is provided fuel bundle for a nuclear reactor comprising a plurality of spacers at axially spaced locations along the fuel bundle and having axially aligned openings, a plurality of elongated fuel rods laterally spaced from one another and extending through the selected openings of the spacers, at least one opening in one spacer along the fuel bundle being devoid of a rod and defining with surrounding rods above one spacer a vent volume, a separation device in the vent volume for flowing liquid laterally outwardly onto the surfaces and into the interstices of the surrounding rods and one opening having a flow area in excess of a flow area through each opening of the spacer containing a rod, the separation device being disposed wholly above one spacer in the vent volume to minimize the pressure drop across one spacer while directing liquid onto the surfaces and into the interstices of the surrounding rods.
In a further preferred embodiment according to the present invention, there is provided a fuel bundle for a nuclear reactor comprising a plurality of rods, including fuel rods spaced laterally from one another in a matrix thereof enabling flow of liquid about the rods from a lower end of the fuel bundle toward an upper end thereof, a plurality of spacers spaced one from the other along the fuel bundle, each spacer having openings for receiving the fuel rods and maintaining the rods spaced from one another in the matrix thereof, each opening defining with a rod through the opening a first flow area for flowing fluid through the spacer, at least one of the rods being a part-length rod terminating in an upper end below the upper ends of surrounding fuel rods and defining with respect to the surrounding fuel rods a vent volume overlying the part-length rod, at least one of the spacers disposed above the partial length rod having an opening therethrough in part defining the vent volume, a separation device disposed above one spacer in the vent volume above the partial length rod for flowing liquid laterally outwardly onto the surfaces and into the interstices of the surrounding adjacent fuel rods, the opening through the one spacer having a flow area in excess of the first flow area for minimizing the pressure drop across one spacer.
Accordingly, it is a primary object of the present invention to provide separation devices for separating liquid and vapor in one or more vent volumes of a nuclear fuel bundle in a manner which minimizes pressure drop along the length of the fuel bundle and supplies additional liquid to the surfaces and interstices of the fuel rods to optimize power production.